There is considerable demand for high performance, low cost, polymer electrolyte materials for use in hydrogen/oxygen and direct methanol fuel cell (DMFC) applications. Several polymer types have been used as proton exchange membranes including, but not limited to perfluorosulfonic acid, sulfonated aromatics, acidified imidazoles, and other organic/inorganic based composites. However, to date, no material has met all the requirements needed to enable fuel cells to become a viable commercial technology. High production costs, high methanol permeability, low physical strength, and/or poor electrochemical performance have plagued the various candidate materials and have hindered the emergence of a suitable material for wide spread use in fuel cell applications.
For the last 30 years, the industry standard proton conducting electrolyte membrane has been Nafion® (polyperfluoro sulfonic acid) produced by DuPont (U.S. Pat. Nos. 3,282,875 and 4,330,654). While the performance of Nafion® is moderately effective as a membrane within the context of hydrogen/oxygen PEM fuel cells, the polymer has a variety of limitations that have hampered the emergence of the proton exchange membrane (PEM) fuel cell design. Among these are Nafion's® high methanol permeability, low thermal stability, and its high cost.
Nafion® is the most commonly incorporated material in all low to medium temperature fuel cells although it works poorly as a direct methanol fuel cell (DMFC) membrane. Nafion's® poor performance within the DMFC context is primarily due to its high methanol permeability and resulting methanol crossover. To minimize crossover, some researchers have incorporated additives into Nafion® as described in US Patent Application No. 2002-0094466A1 as well as volatilized the methanol before introducing it to the anode side of the cell. However, incorporating additives does not mitigate Nafion's® high production cost and the volatilization of the methanol increases fuel cell system complexity.
Other perflourinated sulfonic acid materials have been developed to compete with Nafion®. One alternative membrane incorporates Nafion® or a Nafion®-like polymer into a porous polytetrafluoroethylene (TEFLON®) structure). These membranes are available under the trade name GORE-SELECT® from W. L. Gore & Associates, Inc. and they are described in U.S. Pat. Nos. 5,635,041, 5,547,551 and 5,599,614. Other similar membranes are available under the trade names ACIPLEX® from Asahi Chemical Co. and FLEMION® from Asahi Glass. Regardless of their developer, these alternative membranes exhibit many of the same deficiencies as Nafion®, namely, its high cost and high fuel crossover in DMFC applications.
To address the cost and performance limitations faced with the use of perflourinated sulfonic acid materials, recent research has focused on the development of acid functionalized aromatic polymers for use as proton exchange membranes in PEM fuel cells. Thermoplastics such as polysulfone-udel (PS-Udel) or poly-ether-ether ketone (PEEK) described in U.S. Pat. Nos. 4,625,000, 4,320,224 and 6,248,469 B1, have been extensively studied as ion-conducting materials as described by Tchicaya et. al, Xiao et. al, and U.S. Pat. No. 4,625,000, and U.S. Pat. App. US2002/0091225 (see: Tchicaya, L. Hybrid Polyaryletherketone Membranes for Fuel Cell Applications, Fuel Cells 2002, 2, No 1. and Xiao, G., Synthesis and Characterization of Novel Sulfonated Poly(arylene ether ketone)s derived form 4,4′-sulfonyldiphenol, Polymer Bulletin 48, 309–315 (2002)). Functionalizing these aromatic polymers has the potential of meeting the cost and production challenges that face the perfluorinated based polymers, but has two problematic properties for fuel cell operation: excessive osmotic swelling and low mechanical strength under hydrated conditions.
Aromatic based membranes, such as PEEK, which are described in U.S. Pat. Nos. 4,320,224, 4,419,486, 5,122,587 and 6,355,149 B1, use a post sulfonation process to attach sulfonic acid groups onto the polymer backbone. The frequency of sulfonation or other acid sites improves the electrochemical properties of the ionomer but also increases osmotic swelling and lowers the material's mechanical strength. While the increase in osmotic swelling can help to increase the conductivity of the material, an over hydrated material will become unsuitable for fuel cell applications.
Generally, all of the potentially low cost aromatic based polymers share the same challenges as sulfonated PEEK. Increasing sulfonation (or acid sites) increases the electrochemical performance of the membrane but decreases its mechanical properties after hydration. Many of the aromatic polymers such as the polyether sulfones and polymer aryl ketones as described in U.S. Pat. No. 4,625,000 from Union Carbide Corporation, have significant potential if their weaknesses can be overcome such that they can have high electrochemical properties while also retaining high mechanical properties.
One method that has been employed to overcome the shortcomings of acid functionalized proton exchange materials is to incorporate crosslinking agents. Kerres et al combined sulfinated (—SO2) polymer chains with halogenated alkanes as described in U.S. Patent Appl. No. 2003/0032739 to reduce the osmotic swelling and improve the mechanical strength of the polymer. The reaction linked the sulfinated functional groups of polymer chains via a mid-length alkane, thereby reducing the osmotic swelling of the material. However, this process was experimentally complicated and reduced the proton conductivity of the proton exchange membrane product. Furthermore, the alkane (crosslinking agent) used to crosslink the material was devoid of functionality such as acid sites, and thereby could not add electrochemical performance characteristics to the material.
Another crosslinking strategy that has been implemented to improve fuel cell membrane performance entailed the copolymerization of styrene with divinylbenzene (see Tsyurupa M. P., Hypercrosslinked Polymers: Basic Principle of Preparing the New Class of Polymeric Materials, Reactive and Functional Polymers, Vol. 53, Issues 2–3, December 2002, 193–203.). In this method, the resulting crosslinked ionomer had limited oxidative resistance since both styrene and divinylbenzene display sensitivity to oxidation (Assink, R. A.; Arnold C.; Hollandsworth, R. P., J. Memb. Sci. 56, 143–151 (1993)). The crosslinking of the material did improve its performance characteristics though the base material was susceptible to chemical degradation and thus would limit membrane lifetime.
Accordingly, there is a need for improved proton exchange membrane materials with better physical and chemical properties that have good electrochemical performance such as proton conductivity.